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Full text of "Handbook Of Chemical Engineering - I"

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94                               CHEMICAL ENGINEERING
Discharge of load takes place on the lower run of the conveyor through sliding gates set in the trough. Both long- and short-link chains are employed, the former having the advantage of presenting fewer bearing surfaces (fewer joints, or links), but produce an intermittent tractive effort in passing about the polygonal end sprockets. The advantages and disadvantages in the case of short-link chains are simply reversed. The sprockets in either case are advisably provided with chilled teeth to resist excessive wear.
Conveyor drives should be set in rigid frames with reductions through gearing or by means of silent chains and lubrication should be with a high-grade of medium-body cup grease liberally supplied. Speeds vary from 100 to 200 ft. per minute depending upon the load. The carrying capacity of flight conveyors varies directly with the effective area of the nights, speed of conveyor and weight of the material and indirectly with the spacing of the sights. Inclining a flight conveyor decreases its carrying capacity from 1% to 2 per cent for each degree of angularity. As most flight conveyors are equipped with flights of rectangular form, it will suffice to consider only such conveyors. As the spacing of nights is pretty well standardized at 12,16,18 or 24 in. and the weight of load is a fixed quantity, the potent factor in capacity, other than size of flights and their spacing, is the speed at which the conveyor is run. This varies with the character of the material handled and Table 9 lists the economic speeds for various materials.
TABLE 9.—ECONOMIC SPEEDS FOB FLIGHT CONVEYORS HANDLING VARIOUS MATERIALS
MATERIAL                                                                    SPEED, FEET PER
MINUTE
Coke.............................................            100
Broken stone (coarse)...............................            125
Lump coal, run-of-mine............................            125
Ashes.............................................            150
Lime and cement...................................            150
Ore (average)......................................            175
Crushed stone.....................................            175
Sand and gravel....................................            175
Fine coal..........................................           200
Formulas for ascertaining the capacity of flight conveyors with rectangular flights equally spaced and the conveyors economically and continuously loaded are:
,-r      0.0028AW                           „         .    ,„,
ppA ——______                                    Formula (11)
o
Where,
Wh = Capacity in tons per hour—horizontal conveyor, A — Area of nights in square inches,
w' = Weight of material handled in pounds per cubic foot, and S = Spacing of flights in inches.
Power Requirements.—The consumption of power in operating flight conveyors is materially affected by whether the flights are mounted on sliding shoe or roller flights. Formula (12) by the use of the proper constant permits the calculation of power requirements in either event.
,,                      CWL + WH                           _            _
Horsepower = -----~----                          Formula (12)